1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor integrated circuits, and more specifically to a technique for improving photomasking processes.
2. Description of the Prior Art
The processing of semiconductor integrated circuits includes multiple steps of masking and etching various layers which have been deposited over a substrate and other earlier deposited layers. Photoresist is used for these masking steps. When a layer of photoresist is exposed to light of a proper frequency, such as ultraviolet for example, a physical change occurs in the polymers of the resist. After exposure using a mask as a pattern, the undesired portions of the resist are washed away using a chemical solvent. Those portions washed away may be either the unexposed or exposed portions, depending upon whether a positive or negative resist was used. The pattern of the photomask is thus transferred to the resist, which is used as an etch mask for the underlying layer or layers.
Problems can arise when resist is used in this manner as a mask to pattern layers of materials having a high reflectivity which have been deposited over the device. Metal layers are especially troublesome in this respect. Such metal layers are typically used as metal interconnect layers. Aluminum is a typical metal used for this purpose. A problem results during photolithography because metals such as aluminum are highly reflective, causing the light used to expose the resist to reflect back into the resist and further expose it. Since, in general, the light is not traveling perfectly perpendicularly to the upper surface of the metal layer, it is reflected at an angle to the side of the desired region. This is especially true where the metal layer passes over steps and other uneven topographical features. Since adjacent portions of the resist become exposed by the reflected light, accurate control of feature sizes is extremely difficult when patterning metal layers.
A number of approaches have been used to solve the reflectivity problem, but most such techniques introduce undesirable issues as side effects. It is not possible merely to reduce the intensity of the light used to expose the resist, because this can result in underexpose of some portions of the resist layer. Some techniques have used a thinner layer of photoresist in order to minimize the unwanted side effects from reflection off of the metal layer. However, the thinner resist layer is less effective as a mask for later etching steps. Also, the use of multiple resist layers introduces additional complexity into the process flow. Some techniques for using anti-reflective coatings have been used, but these often introduce additional processing complexity or provide only a partial solution to the problem.
It would be desirable to provide a technique for reducing the reflectivity of a metal layer prior to the deposition and patterning of a photoresist masking layer.